Caesium (symbol Cs, atomic number 55) is the most reactive metal and the basis of the atomic-clock definition of the second. It is classified as a alkali metal and sits in group 1, period 6 of the periodic table.
Overview
Caesium is the chemical element with atomic number 55 and the symbol Cs. With a standard atomic weight of 132.91, it is a alkali metal. In the periodic table it occupies group 1 and period 6, and under standard conditions it is normally a solid. Caesium is the most reactive metal and the basis of the atomic-clock definition of the second.
Atomic structure
An atom of Caesium has 55 protons in its nucleus and, when neutral, 55 electrons arranged in the configuration [Xe] 6s1. Its standard atomic weight of 132.91 reflects the natural mix of its stable isotopes.
Discovery and place in the table
Caesium was discovered in 1860 by Robert Bunsen, Gustav Kirchhoff. It follows Xenon (Xe) and precedes Barium (Ba) in order of atomic number.
| Atomic number | 55 |
|---|---|
| Symbol | Cs |
| Atomic mass | 132.91 |
| Category | Alkali metal |
| Group | 1 |
| Period | 6 |
| Electron configuration | [Xe] 6s1 |
| Phase at STP | Solid |
| Discovered | 1860 (Robert Bunsen, Gustav Kirchhoff) |